I. Introduction
This invention relates to modified light sensitive materials for photoresist compositions. More particularly, this invention relates to photoactive compounds that are thermodynamically stable esterification products of polyhydroxy phenols and o-quinone diazide sulfonyl compounds characterized by improved solubility in organic solvents and at least two o-quinone diazide sulfonyl groups substituted on the phenol.
II. Description of the Prior Art
Photoresist compositions are well known in the art and described in numerous publications including DeForest, Photoresist Materials and Processes, McGraw-Hill Book Company, New York, 1975. Photoresist coating compositions are applied from liquid solution or as a dry film. When coated onto a substrate and exposed to light of the proper wavelength, are chemically altered in their solubility to certain solvents (developers). Two types are known. The negative-acting resist is initially a mixture which is soluble in its developer, but following exposure to activating radiation, becomes insoluble in developer thereby defining a latent image. Positive-acting resists work in the opposite fashion, light exposure making the resist soluble in developer.
Positive-working photoresists are more expensive than negative-working photoresists but are capable of providing superior image resolution. The greater expense of the positive working photoresist is due primarily to the cost of the photoactive compound used to formulate the photoresist. However, notwithstanding the higher cost of the positive photoresist, such materials are in substantial commercial use due to image resolution superior to negative acting photoresists. For example, positive-working photoresist using o-quinone diazide sulfonic acid ester photoactive compound can be developed to yield relief images having line widths of one micron or less. In addition, considering the cross section of a photoresist image, the channels formed in the resist by development have square comers and sidewalls with only minimal taper.
The positive-working photoresists typically comprise a light-sensitive component in a film-forming alkali soluble thermoplastic polymer binder. The light-sensitive compounds or photoactive compounds most frequently used are esters formed from o-quinone diazide sulfonic acids. These esters are well known in the art and are described by DeForest, supra, pages 47-55, incorporated herein by reference. Photoactive compounds and the methods used to make the same are well documented in prior patents including German Pat. No. 865,140 granted Feb. 2, 1953 and U.S. Pat. Nos. 2,767,092; 3,046,110; 3,046,112; 3,046,119; 3,046,121; 3,046,122; and 3,106,465, all incorporated herein by reference. Additional photoactive compounds that have been used in the formulation of positive-acting photoresists are shown in U.S. Pat. No. 3,637,384, also incorporated herein by reference. These materials are formed by reaction of a suitable diazide of an aromatic sulfonyl chloride with an appropriate resin amine. Methods for the manufacture of these photoactive compounds and examples of the same are shown in U.S. Pat. No. 2,797,213, incorporated herein by reference. Other positive-working diazo compounds have been used for specific purposes. For example, a diazo compound used as a positive-working photoresist for deep UV lithography is Meldrum's diazo and its analogs as described by Clecak et al, Technical Disclosure Bulletin, Volume 24, Number 4, September 1981, IBM Corporation, pp. 1907 and 1908. An o-quinone diazide compound suitable for laser imaging is shown in U.S. Pat. No. 4,207,107. The aforesaid references are also incorporated herein by reference.
The resin binders most frequently used with the o-quinone diazide photoactive compounds in commercial practice are the alkali soluble phenol formaldehyde resins known as the novolak reins. Photoresists using such polymers are illustrated in U.K. Pat. No. 1,110,017, incorporated herein by reference. These materials are the product of reaction of a phenol with formaldehyde under conditions whereby a thermoplastic polymer is formed with a glass transition temperature of about 100.degree. C. Novolaks with glass transition temperatures in excess of 100.degree. C. are also known and exemplified in U.S. Pat. No. 5,266,440, which discloses novolak resins comprising the product resulting from the acid condensation of an aromatic aldehyde with a phenol resulting in resins having a molecular weight in excess of 1,500 Daltons and glass transition temperatures in excess of 125.degree. C. Another class of binders used with such photoresists are homopolymers and copolymers of vinyl phenol. Photoresists of this nature are disclosed in U.S. Pat. No. 3,869,292, supra.
In the prior art, the above-described positive resists using alkali soluble phenolic resins as a binder are most often used as masks to protect substrates from chemical etching in photo-engraving processes. For example, in a conventional process for the manufacture of a printed circuit board, a copper-clad substrate is coated with a layer of a positive-working photoresist, exposed to actinic radiation to form a latent circuit image in the photoresist coating, developed with a liquid developer to form a relief image and etched with a chemical etchant whereby unwanted copper is removed and copper protected by the photoresist mask is left behind in a circuit pattern. For the manufacture of printed circuit boards, the photoresist must possess chemical resistance, must adhere to the circuit board substrate, and for high density circuits, must be capable of fine-line image resolution.
Similar photoresists are also used in the fabrication of semiconductors. As in the manufacture of printed circuits, the photoresist is coated onto the surface of a semiconductor wafer and then imaged and developed. Following development, the wafer is typically etched with an etchant whereby the portions of the wafer bared by development of the photoresist are dissolved while the portions of the wafer coated with photoresist are protected, thereby defining a circuit pattern. For use in the manufacture of a semiconductor, the photoresist must possess resistance to chemical etchants, must adhere to the surface of the semiconductor wafer and must be capable of very fine-line image resolution.
To fabricate devices requiring fine-line resolution, it is necessary to use photoresist compositions that are capable of submicron resolution. It is known in the art that high resolution is achieved using photoactive compounds that are mulitesters of naphthoquinone diazide sulfonic acid halides and large polyhydroxy compounds. Though high ester levels of the naphthoquinone diazide sulfonic acid substituent improves image resolution, a high level of esterification may create problems in the manufacture and use of the photoresist. For example, it is known that photoresists formed using multiesters may unexpectedly insolubilize in a photoresist composition resulting in particle formation. Particles cannot be tolerated in photoresists requiring submicron resolution capability. It is also known that the tendency to form particles is greater with photoactive compounds having large molecules and multiple esterified hydroxyl groups. In addition, it is known that efforts to esterify such phenols generally result in a reaction product that is a mixture of photoactive compounds. For example, if a phenol having five phenolic groups is esterified with three moles of a naphthoquinone diazide sulfonyl halide, the resultant product may have an average of about three ester groups per molecule of phenol, but the actual reaction product is likely to be a mixed ester comprising pentaester, tetraester, triester, diester, monoester and unreacted phenol, dependent upon reaction conditions, with the average number of ester groups in the mixture being three ester groups per molecule. It is further known that the higher esterification products, for example the pentaester and tetraester, especially those esterification products having a symmetrical structure, are especially susceptible to particle formation in solution.
Methods proposed in the prior art to prevent particulate formation of a naphthoquinone diazide sulfonic acid halide ester photoactive compound during storage is disclosed in U.S. Pat. No. 4,732,837 and in published Japanese Patent Application H15-83543 published Nov. 26, 1993, both incorporated herein by reference. In accordance with the procedure disclosed therein, unesterified hydroxyl groups--those not reacted with a naphthoquinone diazide sulfonyl halide, are reacted with an acylation or sulfonylation agent thereby chemically inactivating the hydroxyl groups with a concomitant decrease in the tendency of these materials to precipitate from solution during storage. Though it is believed that the process improves shelf life of a photoresist as reported, it is also believed that free hydroxyl groups are important to photospeed and dissolution of unexposed photoresist in developer. Consequently, acylation or sulfonylation of the hydroxyl groups decreases dissolution rate of an exposed photoresist thereby retarding the rate of development or requiring an increase in exposure dose for equivalent development speed.